1. Field of the Invention
The invention is in the field of coherent optical detection techniques. It relates to an optical hybrid and to a coherent optical receiver in which the optical hybrid is used.
2. Prior Art
Coherent optical detection using phase diversity makes baseband detection possible without the need to use `phase-locked loop` (PLL), as is in fact the case for homodyne detection. Since efficient use can be made of the electrical and optical bandwidth in this way, phase diversity is consequently very suitable for multichannel coherent optical communication with high bit rates (.gtoreq.1 Gbit/s). Considerations relating to the suppression of noise, such as thermal noise, also make the use of phase diversity attractive for high bit rates.
To retrieve the amplitude and the phase of a coherent optical signal, phase diversity makes use of optical hybrids. An optical hybrid is an m.times.n multiport having m signal inputs and n signal outputs, where m,n.gtoreq.2, which optical hybrid provides, at two or more of the outputs, coherent products of signals presented at two or more of the inputs and having mutually well-defined phase differences. There are two types of such optical hybrids used for phase diversity. Hybrids of the first type, referred to as the polarisation type, generally produce the desired phase differences between output signals having different polarisations, this being in contrast to those of the second type, referred to as the coupler type, in which the phase differences are produced between output signals having the same polarisation. A number of different variants of the coupler type are known. A first variant is based on the suitable mutual coupling of a number of parallel waveguides in order to produce the desired phase shift of 90.degree.. Such an optical hybrid, in which four optical fibres are arranged in a fused coupling having a square section, is disclosed, for example, by reference [1]. A second variant, disclosed in reference [2], is a similar port comprising a planar strip-like guide having two inputs and four outputs, which is equivalent to four planarly arranged, coupled optical fibres. A third variant, disclosed in reference [3], is a 3.times.3 port which, although related to the first variant in terms of coupling, produced phase differences of 120.degree.. In reference [4] a general S-matrix theory has been developed for reversible 3.times.3 fibre couplers, both loss-free and non-loss-free ones. The theory is applied to an opto-electrical 90.degree. hybrid comprising a 3.times.3 coupler which is equivalent to the abovementioned third variant, which hybrid forms part of a homodyne receiver comprising three photodiodes and a `Costas loop`.
The sensitivity of all these known variants of the coupler-type multiports is the same if all the signals at the output side of such a multiport are used for detection and thermal noise is suppressed as much as possible in the process. However, the use of more than two detectors in this case makes a receiver complex and, moreover, expensive. If, moreover, balanced detection is used, it results in a decreasing receiver bandwidth and an increase in thermal noise. It is therefore desirable not to have to use more than two detectors for the detection. For a two-detector receiver, however, the known optical hybrids are not ideal. In the known coupler-type m.times.n ports having 90.degree. phase difference, the `throughput` is always .ltoreq.25%, that is to say, not more than 25% of the signal power at the input is ever coupled out in each of the outputs used. Since optical hybrids which can be used for phase diversity can also be used for `image rejection` heterodyne receivers, such as those disclosed in reference [4], the same restriction relating to `throughput` applies to such receivers.